Re: ARM unaligned MMIO access with attribute((packed))

[Måns Rullgård]

David Miller <davem@xxxxxxxxxxxxx> writes:

> From: Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx>
> Date: Wed, 2 Feb 2011 16:37:02 +0000
>
>> 1. there's no way to tell GCC that the inline assembly is a load
>>    instruction and therefore it needs to schedule the following
>>    instructions appropriately.
>
> Just add a dummy '"m" (pointer)' asm input argument to the inline asm
> statement.  Just make sure "typeof(pointer)" has a size matching the
> size of the load your are performing.

That should be "m"(*pointer).

>> 2. GCC will needlessly reload pointers from structures and other such
>>    behaviour because it can't be told clearly what the inline assembly
>>    is doing, so the inline asm needs to have a "memory" clobber.
>
> This behavior is correct, and in fact needed.  Writing to chip registers
> can trigger changes to arbitrary main memory locations.
>
>> 3. It seems to misses out using the pre-index addressing, prefering to
>>    create add/sub instructions prior to each inline assembly load/store.
>
> Yes, this is indeed a problem.

GCC has trouble doing anything more complicated than simple indexing.
Load/store instructions with writeback seem not to be in its
vocabulary at all.

> But you really need that memory clobber there whether you like it or
> not, see above.

I don't know of any device where the side-effects are not explicitly
indicated by other means in the code triggering them, so it probably
is safe without the clobber as Russel says.

-- 
Måns Rullgård
mans@xxxxxxxxx

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